In short, this tool allows you to get the hex value of any color from any app and put it into your clipboard. So if you're looking at screenshots in Preview and need to find a certain color blue or red, you don't need to open Photoshop and use the eyedropper. Just open DigitalColor Meter by going to Applications > Utilities > DigitalColor Meter

The first time you use this app, you need to change the default output setting to make this useful for CSS editing. From the dropdown box in the app, select RGB As Hex-Value, 8 bit.

Now, move your mouse over the color you want to grab (in Firefox, Preview, Word, or whatever) and hit cmd + shift + c. This will copy the hex value of the color to your clipboard, and you can then easily paste it into a CSS document.